The FEC system defined in the conventional GE-PON system (IEEE802.3ah (see Non-Patent Document 1)), as shown in FIGS. 8-1 and 8-2, newly defines an S_FEC and a T_FEC. The S_FEC is a code for identification of a frame start. The T_FEC is a code for identification of a boundary between an IEEE802.3 frame (hereinafter, “frame”) and an FEC parity (hereinafter, “parity”) and for identification of a parity end. The S_FEC (/K28.5/D6.4/K28.5/D6.4/K27.7/: Dxx.x indicates a 10-bit data code in the 8B/10B code system) is obtained by extending /S/ (/K27.7/: Kxx.x indicates a 10-bit special code in the 8B/10B code system), which is a frame start identification code in the case of a non-FEC frame. The T_FEC (/T/R/K28.5/D29.5(or /D10.1/)/T/R/ or /T/R/R/K28.5/D16.2(or /D5.6/)/T/R/) is obtained by extending an EPD (/T/R or /T/R/R:/T/ indicates /K29.7/and /R/ indicates /K23.7/), which is a frame end identification code in the case of the non-FEC frame.
These code patterns for frame boundary identification (S_FEC and T_FEC) are added in front of and behind a frame and a parity during frame transmission and outputted to a communication partner apparatus. The receiving unit of the communication partner apparatus establishes frame synchronization (detection of a boundary between the frame and the parity) by detecting these code patterns for frame boundary identification. In IEEE802.3ah, a certain degree of error (in IEEE802.3ah, smaller than 5 bits) is allowed for these code patterns for the frame boundary identification during frame reception to protect code patterns for frame boundary identification not protected by the FEC function and prevent frame synchronization from being overlooked even when a frame propagates on a transmission channel with a high error rate.
Non-Patent Document 1: IEEE802.3ah, section 65.2.3